In general, in order to send real-time video (or multimedia data) over wired/wireless Internet protocol (IP) networks, conditions such as a sufficient bandwidth of a channel, small delay, minimum packet loss, and the like must be satisfied. However, a network layer on current wired/wireless IP networks does not provide appropriate functions for satisfying quality of services (QoS) which are required for a video transmission.
Therefore, the QoS must be ensured by a higher layer of the network layer. A real-time transport protocol (RTP) and a real-time transport control protocol (RTCP) which are operated on a transport layer have been proposed. By using the RTP and RTCP, characteristics according to time limit can be considered, and it is possible to adaptively deal with a loss occurring within a network.
Since the RTCP provides to a sender (sending node) information of a current network condition and a reception QoS, the information provided can be used such that the sender automatically adapts to the network condition and the reception QoS to regulate a transmission rate, while a network manager measures a multicast performance.
FIG. 1 illustrates a structure of a sender report (SR) packet of the RTCP. FIG. 2 illustrates a structure of a receiver report (RR) packet of the RTCP. The SR packet is sent from the sender to the receiver when multimedia data is sent. The SR packet can be used when the sender performs sending and receiving at the same time or when sending. The RR packet is sent from the receiver to the sender when the multimedia data is received.
The SR packet and the RR packet include reception report blocks, respectively, each of which includes statistical information of RTP packets sent by one sender and feedbacks the statistical information from the receiver to the one sender. The SR packet further includes a packet length, sender information, sending time information, sender's packet count, and sender's octet count.
Each of the reception report blocks further includes information of such fraction loss, a last SR timestamp (LSR) and a delay since last SR (DLSR). The fraction loss denotes a packet loss rate, the LSR denotes 32 bits which are parts of 64 bits of a network time protocol (NTP) timestamp of a currently-received RTCP SR packet, and the DLSR denotes a delay time since the last SR packet.
The DLSR denotes, by ½32 ( 1/65536) second unit, the delay time until the reception report block of the RR is sent after the last SR packet of the sender is received. The time information associated with the packet sending such as the LSR, the DLSR may provide important information to estimate a transmission rate of the multimedia data to be transmitted later by the sender.
In a typical method for estimating an effective transmission rate using status information of a network, the RTT is used, and the RTT is measured by the SR and RR of the RTCP. The sender sends the SR which includes the NTP timestamp value when the message is sent. The receiver remembers the time of having received the SR. As a result, when the receiver sends the RR to the sender, the receiver records the time interval from the time of having received the SR and to the time of sending the RR in a DLSR field of the RR packet.
The receiver records information of partial bits of the NTP timestamp value of the received SR in an LSR field of the RR packet, and thereafter sends the RR packet to the sender. The sender checks its reception time of the RR packet. Thus, the sender subtracts the received DLSR field value and LSR field value from its reception time of the RR packet so as to measure the RTT.
Because the RTT is an instant RTT obtained from the RTCP SR packet information or the RTCP RR packet information, it is sensitive to instantaneous changes in a channel's mode. Therefore, when the effective transmission rate is estimated by adopting the RTT which is drastically changed according to the instantaneous change of the channel circumstance, the effective transmission rate also has severe fluctuation.
In the VBR multimedia sending system such as a VBR video sending system, a drastic fluctuation of the effective transmission rate causes a severe variation of image quality or sound quality. As a result, a great load is generated in controlling a buffer for a network transmission. Thus, the multimedia quality delivered is degraded.